**Question:** 1st carbon of pentose sugar of nucleic acid joins

A. 3' end
B. 5' end
C. Phosphate group
D. Base pair

**Core Concept:**
Nucleic acids are biomolecules essential for genetic information storage and transmission. They consist of nucleotides, which are composed of a phosphate group, a sugar (deoxyribose or ribose), and a nitrogenous base (adenine, thymine, guanine, or cytosine). In DNA (deoxyribonucleic acid), the sugar is deoxyribose, while in RNA (ribonucleic acid), it is ribose.

**Why the Correct Answer is Right:**
The first carbon of the pentose sugar (deoxyribose or ribose) in nucleotides of DNA and RNA respectively, forms a covalent bond with the phosphate group. This sugar-phosphate-nucleotide structure forms the backbone of the nucleic acid molecule.

**Why Each Wrong Option is Incorrect:**
A. Option A, 3' end: The correct position for the covalent bond between the sugar and phosphate group is the 5' (5'-phosphate) end, not the 3' (3'-hydroxyl) end.
B. Option B, 5' end: The correct position for the covalent bond between the sugar and phosphate group is the 5' end, not the 5' end.
C. Option C, Phosphate group: Although the phosphate group is essential, it is not the carbon to which the sugar is covalently bonded.
D. Option D, Nucleotide: While a nucleotide is a basic unit of nucleic acids, it is not the correct answer because it refers to the overall molecule, not the specific carbon in question.

**Clinical Pearl:**
Understanding the correct positioning of the covalent bond between the sugar and phosphate group is crucial for understanding nucleic acid structure and function. This knowledge is essential in understanding DNA replication, transcription, and genetic processes. For example, in DNA replication, the 5' end of the DNA strand serves as a template for the formation of the new complementary strand, while the 3' end remains on the original strand.